DETAILED DESCRIPTION OF THE INVENTION
Multilayer ceramic modules are in widespread use in microelectronic technologies. In their fabrication, an organic binder, solvents and ceramic particles are cast into thin sheets. The sheets are dried, punched with holes to provide access for conduction paths and screened with a metal paste to form the conduction paths for electrical circuits. Most commonly, the materials used are alumina powder, poly(vinylbutyral) binder and tungsten or molybdenum metal lines for circuitry.
The green sheets are stacked and laminated under pressure to form a green module. The green module is then fired to remove the binder and to sinter the particles into a miniature brick. During the initial stages of firing, that is, at temperatures below 600.degree. C. (873K), the binder volatilizes and decomposes producing a carbonaceous residue at the surface of the ceramic particles and metal particles. This debinding stage is critical in the formation of sound ceramic parts. Often internal pores and crack are formed during the debinding due to the internal pressures generated by the evolving gases. Heating rates during the debinding stage must be low enough to avoid bloating and cracking. The required debinding times are often several hours for thicker parts.
Most of the carbonaceous residue must be removed to achieve desirable electrical and mechanical properties in the module. As the carbonaceous residue level increases, the dielectric constant of the substrate increases resulting in increased signal delay times. At high carbonaceous residue levels, occasional shorts occur between the intended conduction paths.
The carbonaceous residue level is affected by processing variables-significantly, by the composition of the atmosphere in the firing process during which debinding occurs. Oxidizing atmospheres at the higher firing temperatures proximately below the temperature at which the pores in the ceramic structure begin to close, about 1270K, produce lowest levels of carbonaceous residue. However at these temperatures, the metal circuitry is also subject to oxidation. Thus at these higher temperatures in the firing, the atmosphere is desirably reducing or neutral with respect to the metal and oxidizing with respect to the carbonaceous residue.
Such a mild oxidizing atmosphere has been provided by water vapor in hydrogen, often diluted by an inert gas, usually nitrogen. FIG. 1 depicts as a function of temperature the hydrogen-to-water ratio for equilibrium between carbon and carbon dioxide; between tungsten and tungsten trioxide; and between molybdenum and molybdenum dioxide. As can be seen, above approximately 973K for tungsten, and above approximately 1073K for molybdenum, it is possible to provide an atmosphere of hydrogen and water which will be reducing or neutral to the metal and also oxidizing with respect to carbon.
Sohn and Wall in their publication entitled "Removal of Carbonaceous Residue with Wet Hydrogen in Green Sheet Processing of Multilayer Ceramic Module: II, Reaction in Large Modules Influenced by Pore Diffusion and Mass Transfer" published in J. Am. Ceram. Soc., 73 (10) 2953-61 (1990) suggest continuously adjusting the water and hydrogen concentration as a function of firing temperature of the module to maintain the equilibrium value for molybdenum oxidation. While this criterion is useful to establish the hydrogen-to-water ratio in a mixture of hydrogen and water alone, it does not provide guidance as to appropriate levels of hydrogen and water in admixture with an inert gas.
Typically water is introduced into the gaseous atmosphere by bubbling a hydrogen and nitrogen gas mixture through liquid water at a given temperature. Such coarse control of the water oxidant is difficult and imprecise, however, resulting in variable levels of carbonaceous residue and at times oxidization of the metal circuitry. Furthermore, the composition achieved at the entry of the gas into the debinding furnace may not be that desired at the reactive site itself. In addition, altering the composition of an atmosphere at one or more points in a furnace is operationally difficult where the atmosphere is created by bubbling gas through liquid water.
The debinding process, to avoid deleterious results such as bloating, cracking, low densification, warping, and delamination, is usually conducted at a low rate of heating. Typically the rate of heating does not exceed 5 K/min, which causes undesirably long processing times.
An advantage of this invention is that low carbonaceous residue levels are achieved in the firing process.
Another advantage of this invention is that higher rates of heating without damage to the product are achieved in the debinding process resulting in shorter processing times.
Still another advantage of this invention, is that closer control of the firing atmosphere is achieved resulting in a product of less variability in properties.